Research

Language comprehension is fundamental to everyday life. However, underlying the apparent ease with which most engage in linguistic activities are a host of complex processes. These processes - perceptual, memorial, and linguistic - are supported by (and coordinated by) myriad areas of the brain. The main goal of my research is to study the cognitive and neural architectures of normal language comprehension. In this context, my research has focused on a number of topics, including:

1. Coreferential processing
Comprehenders engage in coreferential processing in order to track, "who does what to whom" in written and spoken discourse. Two expressions are coreferential when one (the anaphor) refers to an earlier mention of the same entity (the antecedent). The ability to establish coreference is critical to establishing and maintaining the coherence of written and spoken discourse. In the absence of successful coreference, sentences such as (1) and (2) below, to say nothing of most narratives, would be largely opaque. The name of any person (or object, or event) would be interpreted as introducing a unique entity, and pronouns (like "he") would be effectively worthless.

(1) Bill handed John some tickets to a concert but Bill took the tickets back immediately.

(2) Bill handed John some tickets to a concert but he took the tickets back immediately.

Coreferential processing involves a number of operations: comprehenders must establish entities in memory, such as Bill and John; they must recognize that a word is an anaphor, which may take various forms, including pronouns, repeated names, and definite noun phrases; they must determine the anaphor's unique referent; and they must link the anaphor to the antecedent. All of this must be accomplish rapidly, before the new input begins to interfere with previously received information. In fact, recent evidence suggests that the search for an anaphor's referent begins within 180 ms of encountering the anaphor. Interestingly, in most studies of coreference, readers comprehend a series of unrelated sentences describing simple actions or events involving one or more fictional characters (e.g., "Mary and Jane had an argument when they drove home from work."). The characters described in these sentences (e.g., Bill, John) rarely have "real-world" reference; that is, they do not refer to actual people. Using a diverse array of methods (including event-related potentials, eye tracking, probe verification, and self-paced reading) I have shown that having knowledge about discourse entities has consequences for sentence processing.

2. Individual differences in discourse comprehension and representation
Reading is a complex, intellectual skill, consisting of multiple component processes. The ability to learn and execute these processes varies widely, and large individual differences in reading comprehension are found even among college students. While reader characteristics like working memory capacity, vocabulary skill, and phonological ability have been extensively investigated, individual variation alone cannot explain the variability in comprehension performance. The nature of the material that is read exerts a significant influence on the comprehension process, the complexity of the language and the length and genre of the text. Individuals and materials interact in every aspect of language comprehension.

I have conducted research examining individual differences in comprehension ability and in memory for discourse. My colleagues and I argue that the processes involved in constructing discourse representations are reflected in readers' memory representations. Against this backdrop, we have advocated the use of the remember/know paradigm (as opposed to more standard recall and recognition memory measures) as a tool to assess readers' discourse representations. Those processes that are involved in extracting individual ideas from a sentence are likely to yield memory representations that are retrieved during recognition on the basis of familiarity. In contrast, processes that are involved in integrating ideas with each other and with world knowledge are more likely to yield memory representations that are retrieved during recognition via familiarity. Moreover, individual variation in comprehenders' reading skill and relevant background knowledge about a text modulate both comprehension and memory for discourse.

3. Hemispheric differences in language processing
It is well known that, in most humans, the left cerebral hemisphere (which contains both Braca's and Wernicke's Areas) is primarily responsible for language processing. In contrast, the contributions of the right hemisphere to everyday language processing remain unclear. The involvement of the right hemisphere seems to increase with the complexity of linguistic stimuli, being the least activated at the level of the word, and maximally activated at the level of narrative discourse. However, clinical studies show that, while patients with left hemisphere damage may develop severe language impairment, the deficits that may result from right hemisphere damage tend to be subtle in their presentation, emerging primarily in cognitively sophisticated settings such as narrative comprehension and conversation. The disparity between the consistent, widespread activation of the right hemisphere observed during normal language function and the high level of linguistic function retained following right hemisphere damage is something of a paradox.

I have conducted research investigating how the two cerebral hemispheres organize the representations of discourse. This research has relied on the Item-Priming-in-Recognition paradigm, whereby participants read short passages and then verify whether they saw probe words during reading. These words were prime-target pairs, where the prime was either in the same proposition, the same sentence, the same passage, or a different passage altogether. Primes are presented centrally, while targets are presented to either the left visual field (reporting first to the right hemisphere) or in the right visual field (reporting first to the left hemisphere). Our results have shown that the left hemisphere represents discourse according to propositional relations, with the fastest reaction times to same proposition pairs, and the slowest to targets primed by words from different passages. In contrast, the right hemisphere shows no such structuring. Instead, it seems to represent words that it "sees" together equally well, something like a "bag of words". Primes that appear on the same screen as the target words facilitate similarly regardless of structure (and even in the absence of structure, when sentences are scrambled). Primes that appear on a different screen do not facilitate reaction time even when from the same passage (i.e., when a passage's sentences appear on different but successive screens).